I have had a Marshall DSL 40 amp for about a year. Lately sometimes when playing the high e string about half-way up the fingerboard, I get a very high treble frequency that is very loud and ear-piercing. Last night I played for about two hours and at hour one and a half the amp started sounding very distorted and fuzz toned. That went on until I stopped playing. Also sometimes when I first power-up the amp there is a pronounced backround noise that usually quiets down as soon as I start to play. My question is could the power tubes be going bad? Any technical advice would be greatly appreciated.
Could be, only way to know is to replace them. It can't hurt can it? Yes maybe it will cost some money but if it's not the tubes then you always have an extra set. Eventually you will have to replace them anyways. Probably not the power tubes though, pre-amp tubes generally create issues like this. You can tap on the tubes and if you hear ringing or other weird issues then they are probably bad. Tube amps are very finicky. It could be bad caps or some resistor out of spec. Some amps are just poorly designed and have these weird problems.
Heat is a big deal, when components heat up they change their behaviors, resistors become more noisy, capacitors can change how they work(which changes their filtering behavior), etc. Tube amps generally can get very hot(and poorly designed amps will not deal with the heat in a proper way).
Get you a full set or two of replacement tubes and go from there. You don't have to get expensive ones, in fact cheap ones generally are ok as long as they are relatively popular. You can buy tubes that are bad, shipping can damage them, etc.
Also, you can swap tubes around. Generally the first pre-amp tube amp has the most issues because he does the most amplification, but sometimes this is not the case because it is just a buffer/follower(which just produces the same signal but with a high impedance input, low impedance output so the signal can be used for something). Some tube amps use jfets and mosfets which can fail in weird ways.
You are going to have to replace the tubes one day and they don't age sitting in a closet so might as well go ahead and buy some(inflation will only make them more expensive so you can think of it as an investment).
If it is not the tubes, then, of course, you have other problems but a good chance it is the tubes so might as well go with it, again, it can't hurt.
Also, you can mix and match tubes. Generally one wants a balanced pair for output tubes because one has a push pull configuration, which is a symmetric arrangement so each tube needs to work just like the other. They are suppose to be balanced at the factory but not always. Of course it's actually not a huge deal with an amp and the distortion produced is not always bad(depends on sound and all that).
Also, if you have 4 output tubes you can usually run just 2, this allows you to mix and match, but you have to have the right 2 in(they act as a pair) or you will get no sound and could damage the amp. (also generally you want to make sure your speaker is plugged in when on because you have to load the OT transformer so the tubes are properly loaded)
Tube amps are finicky, as I said, all kinds of weird problems. They sound great when working their best but you need to do some research about them and I suggest you learn to read a schematic and a little electronics so you have some idea whats going on, it won't hurt in any way and you'll know more about stuff.
Yes, tubes can go bad.
Are there more than one of the SAME tube in the amp? Typically a couple of ECC83 or EL84? If so, swap them. Try the power tubes first (that's the EL84). It may not cure the problem, but does it now sound different? That will help pin it down. Have you got a friend with a similar amp? Perhaps he'll let you borrow his tubes.